The state of the art for sealing laser beam paths in semiconductor processing equipment entails enclosing the entire optics volume with a cabinet style enclosure. Some designs incorporate a purging system using some sort of clean dry air or inert gas. Beam tubes are also used in other laser applications outside of micromachining, such as integrated circuit error correction. Most recent designs of UV optic rails and beam paths use covers to protect the optical components. Neither purge gas inside the enclosed volume nor beam tubes with purge gas have been used in laser micromachining applications.
It is well known in the laser industry that UV wavelength laser light can be very damaging to system optical components. The photon energy given by E=hν(where h=Planck's constant and ν=optical frequency) for UV light is sufficient to break and reform bonds in many common airborne molecular contaminants (AMCs). In this process known as photopolymerization, polymers are formed on optical surfaces that intersect the laser beam. The polymers cloud the lenses and mirrors, reducing optical transmittance of the system, and causing beam distortion that degrades performance. Similar problems may occur in the presence of particulate contamination. Particulates can become vaporized, and in turn, polymerized onto optical surfaces. Additionally, in the presence of high instantaneous energy pulsed beams, an acoustic “shock” wave may be formed as a particulate is ablated. This acoustic shock wave may damage optical coatings, substrates, or both, as it propagates into an optical component.
Currently available pulsed lasers with nanosecond, picosecond, or femtosecond pulse widths suffer optical degradation resulting from the high peak powers incident upon their optical components. Often, based upon the application, the laser may deliver excess output energy that must be attenuated. Currently available attenuators, often composed of a half-wave plate and polarizer combination (or a variation of this theme), are inserted into the path of the laser beam to attenuate the laser beam by manipulating its polarization state. Although the technique of using a half-wave plate and polarizer offers the ability to adjust the level of attenuation, the attenuator assembly usually must be placed after several optical components “downstream” from the laser output. The reason is that the half-wave plate and polarizers work best when collimated or nearly collimated light is incident upon them. In addition, the half-wave plate, in the case of a sealed laser rail, would not make a very good window into the sealed portion because waveplates are prone to contamination, are fragile, and are temperature sensitive.
A laser rail, forming part of a laser optical system and sealed from the outside environment, uses input and output windows of the optical system to allow the beam to pass into and out of the sealed portion of the laser rail. Moreover, it is desirable to decrease the amount of laser light incident on all optical components because the intensity of the laser light (in W/cm2, peak W/cm2, or J/cm2) is proportional to the age of the optics. Therefore, in an ideal laser system that produces excessive laser power, the very first component in the optical system would be an attenuator of some type. In summary, it would be desirable to provide the same optical element(s) functioning as an input window and an attenuator.
Laser optical systems include laser shutters that can be divided into two different categories. They include modulation, exposure, and pulse gating shutters and safety interlock and process control shutters. Safety interlock shutters, which are of interest here, intermittently block the laser beam by means of a material that is opaque to the laser wavelength and is caused to move selectively in and out of the line of propagation of the laser beam. The blocked laser beam is reflected into or onto a laser beam “block” or “dump,” which serves to absorb and attenuate the blocked beam. Shutter actuation devices include, but are not limited to, electro-mechanical (solenoid), electrical, and magnetic devices.
A shutter operating as a safety (rather than a modulation) device opens and closes at a low frequency of repeated operation (<<1 Hz). The open and closed positions are sensed and fed back to the operating system. A properly designed laser shutter blocks laser emission and does not cause it to reflect back into the lasing cavity. Shutter construction materials should be free from components that are likely to contaminate the optical system.